Storage drives wear out over time. For example, both flash memory cells in a solid-state drive (SSD) and spinning disk sectors in a mechanical hard disk drive (HDD) can degrade or otherwise become unusable over time. The average time it takes for a storage drive to wear out to the point of failure is called the Mean Time Before Failure (MTBF). Techniques to extend the useful life of a drive increase its MTBF.
Spinning disk sectors and flash memory cells degrade or become unusable in a relatively unpredictable manner. The failure of one sector or cell does not necessarily imply the same for others. For example, magnetic sectors from spinning disks may be written to or read from many times without a given limit, and, without warning, may fail to process a write or read properly. In the case of flash memory, cells wear out on average after a given number of write “cycles,” in addition to other unpredictable and sudden failures.
To compensate for these failures, storage drive manufactures can overprovision the storage drive with spare groups of sectors or cells called blocks. Instead of counting the spare blocks toward the stated capacity of the storage drive, the storage drive reserves the spare blocks to increase its MTBF. In the case of memory that wears out over time such as flash memory, the spare blocks may also be used for wear leveling. Wear leveling spreads write commands to the storage drive more evenly across all of its blocks to increase the time it would take for any one block to fail. As blocks do fail, the spare blocks may be used to replace the failed blocks. The bigger the pool of spare blocks, the longer a storage drive is likely to function according to its MTBF. Due to the unpredictable nature of these failures, individual storage drives deplete their pools of spare blocks at different times, even if the drives are all part of the same storage pool, e.g., Redundant Array of Independent Disks (RAID). Any drive failure in a storage pool poses the risk of potential data loss. Even in redundant or parity-based storage pools, any drive failure in a storage pool poses a potential data loss risk due to a reduction of fault tolerance until the drive is replaced in the pool.
In view of the foregoing, it may be understood that there may be significant problems and shortcomings associated with conventional technologies for managing spare blocks in a storage drive pool.